Hotmelt pressure sensitive adhesives (hot melt PSAs) are compounds which combine the properties of hotmelt adhesives with those of pressure sensitive adhesives. Hotmelt PSAs melt at elevated temperatures and cool to form a permanently tacky film which flows adhesively on contact with a substrate. In combination with various substrates, such as paper, fabric, metal, and polymer films, for example, it is possible to produce a large number of different products, particularly pressure sensitive adhesive tapes and also labels. These pressure sensitive adhesive products have a broad field of application in the automobile industry, e.g., for fastening or for sealing, or in the pharmaceutical industry, for active substance patches, for example.
The typical coating temperature for hotmelt PSAs lies between 80 and 180° C. In order to minimize the coating temperature, the molecular weight of the hotmelt PSA to be applied should be as low as possible. On the other hand, the PSA must also possess a certain level of cohesion, so that the PSA tape does not slip from the substrate in use. In order to increase the cohesion, in turn, a high molecular weight is essential.
In order to solve this problem polymers have been developed which possess a relatively low molecular weight but contain double bonds along the side chains. These polymers, such as polyester acrylates or polyurethane acrylates, for example, can be crosslinked efficiently via the double bonds using UV or ionizing radiation, but have only limited adhesive properties.
In acrylic PSAs, crosslinking is promoted by adding polyfunctional acrylates and/or methacrylates prior to crosslinking, which raise the crosslinking reactivity and hence also increase the cohesion, but which react only by way of a two-stage mechanism during irradiation (attachment to the polymer and then crosslinking reaction by way of the acrylate double bond which is still free) and therefore have a low crosslinking efficiency.
The principle of functionalizing double bonds by copolymerization cannot be employed analogously for acrylic PSAs, since in that case the corresponding polyacrylates are prepared by free radical polymerization. All of the double bonds here are reacted in the polymerization process, or instances of gelling occur during polymerization. One example of this was depicted by Pastor [U.S. Pat. No. 4,234,662 A], who used allyl acrylate or allyl methacrylate for the polymerization. A central problem, however, lies in the copolymerization of these compounds, which generally gel during the free radical polymerization process. Moreover, owing to the relatively low reactivity of the allyl groups in respect of a crosslinking reaction, drastic experimental conditions are necessary, in particular high temperatures or a long period of irradiation. For application as a crosslinked PSA, therefore, the allyl-modified acrylic polymers are not very suitable.
Another possibility for functionalization of double bonds exists by virtue of polymer-analogous reactions.
Generally speaking, polymer-analogous reactions can be conducted in solution or from the melt. EP 0 608 981 B1 likewise refers to the gelling problem with double bonds. This is assisted by diverse further polymer-analogous reactions. Accordingly, polyacrylates with carboxylic acid, hydroxyl, epoxide, and amine groups can be reacted in a polymer-analogous reaction with compounds containing double bonds; in this regard see U.S. Pat. No. 4,665,106 A. Owing to the low thermal stability of the components involved, however, it has not been possible to apply this reaction to hotmelts. Moreover, operating conditions were disadvantageous owing to the fact that in order to avoid gelling it was necessary to add large amounts of regulator to the polyacrylate.
For acrylic hotmelts, therefore, U.S. Pat. No. 5,536,759 A describes the reaction of polyacrylates containing hydroxyl or carboxylic acid groups with 1-(1-isocyanato-1-methylethyl)-3-(1-methylethenyl)benzene (m-TMI) in solution with subsequent hotmelt processing. It is in contrast to this that the pros and cons of the individual processes are described (Chemie Ingenieur Technik (70), 1998, pp. 560-566); “Polymer-analogous reactions in the melt permit two processes which otherwise proceed separately from one another. First of all, the reaction takes place; since the reaction medium is the melt, shaping by extrusion can be commenced during the reaction. In this way, no additional reaction vessel and no work-up at all are necessary. Nevertheless, the absence of the solvent complicates the course of the reaction in a variety of respects; for example, by the heterogeneity of the reaction mixture and the relatively slow diffusion of the reactants into one another”.
Accordingly, the process described in EP 0 608 981 B1 displays the fundamental disadvantages of a polymer-analogous reaction in solution. What would be desirable, therefore, would be a process for acrylic PSAs which allows polymer-analogous reactions in the melt.
A central problem lies in the slow diffusion of the reactants. This problem can be solved only by raising the reaction temperatures, which improves the reactivity of the individual components with one another. For acrylic PSAs, however, there are natural limits on this.
For polymer-analogous reactions in the melt, therefore, the materials used are generally thermoplastics, which are processed and functionalized at high temperatures. For example, polystyrene-maleic anhydride thermo-plastics are reacted at temperatures of 180-200° C. [Chemie Ingenieur Technik (70), 1998, pp. 560-566 and Chemie Ingenieur Technik (71), 1999, pp. 1418-1421]. Additionally, polyesters are reacted with maleic anhydride in the melt [Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 37, 1693-1702 (1999)]. Both processes, however, are unsuitable for the functionalization for acrylic PSAs with double bonds. Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 37, 1603-1702 (1999) discloses functionalization by radical grafting, but this cannot be used for functionalization with double bonds on account of the fact that the vinyl compounds would immediately be polymerized and so would no longer be available for subsequent crosslinking on the backing. The prior art uses polymers [Chemie Ingenieur Technik (70), 1998, pp 560-566] whose glass transition temperatures are too high for PSAs and, if applied analogously to acrylic PSAs, would exhibit excessively high reaction temperatures (at the high temperatures employed, severe discoloration of the polymer already occurs, owing to reactions by, for example, thermally decomposing initiators which have remained after the polymerization process or by the decomposition of individual copolymers, such as tert-butyl acrylate, for example, at above 160° C., and also possess very high fractions of copolymerized maleic anhydride, which places the glass transition temperature very high. The process described in U.S. Pat. No. 5,536,759 A for reacting polyacrylates with 1-(1-isocyanato-1-methylethyl)-3-(1-methylethenyl)benzene (m-TMI) in solution can also not be applied analogously for the processes described above, since in addition to the high toxicity of the isocyanates the crosslinking reactivity after coating would be too low.
A serious and general disadvantage of all of the processes described so far lies in the low crosslinking reactivity after coating. Vinyl compounds have a low reactivity toward the radicals that are generated for crosslinking, with the consequence that crosslinking is incomplete and not very effective. Competing reactions which do not lead to the desired crosslinking, such as saturation of the radicals produced by atmospheric oxygen or added tackifier resins, predominate. The poor controllability of crosslinking may therefore be very problematic, for example, for the aging behavior of the PSA, since unless all of the double bonds are consumed by reaction during crosslinking, the PSA will have a potential for post-crosslinking on prolonged storage and also, under the influence of ultraviolet light or oxygen and/or ozone, will react and undergo a marked loss of bond strength.
There is therefore a need for compounds which can be reacted very quickly and without gelling in a polymer-analogous reaction and for a process operation which allows gel-free processing and coating on a backing, with the aim of obtaining new kinds of acrylic PSA tapes functionalized by reactive double bonds, which can be crosslinked with high reactivity by actinic radiation.